Frank did not have time
Loved this movie, my only problem was that Frank had too much to do and too little time.
After he watches Cheri drive off in the Honda he walks by the TV and turns it off then we get a close up of a clock that says 11:04 (10 minutes before he throws the body off the bridge).
This means he takes the dog out, finds the body, pulls off the stone head, buries the head, fetches Aarons car, gets the body, drives to the bridge and throws it over all in 10 minutes time. In real time it took 9 minutes and they skipped a whole lot.
Only if the clock in his livingroom wasn't accurate this could've happened.